This invention relates to sign posts, and more particularly to sign posts which are adapted for supporting highway signs. Due to the widespread use of signs along highways, various governmental bodies have developed standardized criteria for highway signs. Factors considered important in the development of those criteria are: the design loadings, design life of a post exposed to the elements, initial cost, replacement cost, versatility, installation method, and vehicle speed change.
The design loadings are figured from wind pressure on the sign and wind buffeting. The method of installation may also generate significant stresses. Versatility is demonstrated by posts that may be used in combination to support larger signs, or by posts that are able to support more than one sign per post. Safety is at present usually defined in terms of deceleration time to protect vehicle occupants, and a qualitative assessment of affects on vehicles under body damage, and front end failure.
The vast majority of the present applications for highway sign posts are filled by steel "U" channel posts or steel tubes. Standard sections of these steel channel posts are approximately 2 inches deep and 3.5 inches across, and weigh 3 pounds per foot. Applications for lighter signs are filled by a similar steel section with a weight of 2 pounds per foot. These steel sections can be drilled or punched with holes at standard locations during the manufacturing process so that signs can simply be bolted to the post in the field. Installation is normally accomplished by driving the post into the ground, or by digging a post hole. Steel posts withstand weathering for up to 15 years before they require replacement.
One problem with the present art of highway sign posts is that during a vehicle collision, the steel post will yield upon impact and will bend over the hood of the automobile. Because the post remains rigid and deformed even after this yielding failure it can penetrate the windshield. This causes severe damage to the automobile, loss of control by the driver, and many times results in injury to the vehicle occupants. The improved fiberglass (F.R.P.) or fiber and resin composite highway sign post of this invention is designed to reduce this hazard of highway travel by more precisely controlling the failure mode of the sign post, by selecting a component and post configuration combination that gives the desired failure mode and location. The high strength, lighter weight, and failure mode imparted by the fibers of a tubular fiber and resin construction relative to the prior art steel post provide for a sign post that will fail in shear, or in equivalently safe mode, at or near the impact point upon high speed impact by a moving body. A natural shear plane can also be provided by abruptly varying the section properties of the post at a location that will be near the impact point of a vehicle bumper when the post is installed. The composite sign post of this invention is designed so that in a slow speed collision the post will fail near the ground connection and lay flat, and in a very high speed collision the post will fail near the impact point and the upper portion will fly up over the vehicle. Both modes of failure do minimal damage to the vehicle.
Prior art patents of relevance are: U.S. Pat. No. 4,092,081 (Schmanski), U.S. Pat. No. 3,820,906 (Katt), and U.S. Pat. No. 3,709,112 (Ebinger). Schmanski discloses a marker post designed to flex and lay over upon impact and then to return to its upright position. These marker posts would not have the structural properties of the closed tube design of the present invention. Consequently, unless completely redesigned or made inordinately heavy, they would not support a sign subjected to the expected loadings. In addition, a post built to highway sign height would appear to pose the same threat to windshield penetration since at high impact the Schmanski post could lay over the hood of a colliding vehicle. The Schmanski post is designed to withstand relatively large axial loads during its installation by driving. In the wrapped over position the upper end of the Schmanski post could penetrate the windshield and injure the occupants.
The Katt patent discloses a standard steel channel sign post of two segments that is provided with a frangible connection to connect a lower segment of the post to an upper segment of the post. The Katt disclosure specifically states a limitation to open channel steel type posts and its multiple components apparently must be assembled in the field or greater care must be taken during installation to prevent damaging the connection. The problem addressed in Katt is that of securing the upper segment from being projected due to impact from a vehicle while providing a specific failure mode. The need for securing does not present itself with the post of this invention because of the much lighter weight of the post and the resulting reduced threat of vehicle damage. It further is not apparent that upon a high speed impact the steel channel of Katt could not deform and lay over the hood of the impacting vehicle, with the likelihood of serious injury to the occupants. Although stressed to yield, the Katt post remains stiff and bent. The stiffness integrity of the Katt post is not destroyed as it is in the composite post of the present invention.
Ebinger discloses another guide post of flexible construction that operates similar to that of Katt. There is no provision for allowing but controlling the failure of the posts as there is in the present invention The Ebinger post is flexible to begin with and remains that way. The post herein is rigid to begin with, but the stiffness is destroyed by collision.
For a given Weight per foot, the improved sign post of this invention is many times stiffer in torsion than the standard steel post, because of the cross-section utilized. The versatility is therefore improved for a given weight per foot because the post of this invention will support a larger sign or a larger number of signs. The weathering ability of the post of this invention is on a par with that of the standard steel post when coated. Installation of the post of this invention is by driving or drilling as some of the standard posts are installed, or by utilizing a ground anchor socket. The cost of initial installation in the augured hole applications will be similar to that of a standard post, while the cost of replacement installation will be less than the standard steel post in the applications utilizing an anchor socket because of the controlled failure mode.